1. Field of the Invention
The present invention relates to a magnetic recording medium and production thereof. More particularly, it relates to a magnetic recording medium comprising a magnetic layer containing chromium oxide, which has improved long term storage stability.
2. Description of the Prior Art
It is well known that chromium dioxide (CrO.sub.2) powder has a ferromagnetic property and is used as a magnetic powder of a magnetic recording medium. Chromium dioxide powder is commercially produced by oxidation of Cr.sub.2 O.sub.3 and CrO.sub.3 according to following reactions: ##STR1##
Coercive force and/or particle size of the chromium dioxide powder can be adjusted by the addition of Sb.sub.2 O.sub.3, Fe.sub.2 O.sub.3, TeO.sub.2, RhO.sub.2, IrO.sub.2 or the like to produce magnetic powder for magnetic recording. Recently, the amount of the chromium dioxide powder consumed as the magnetic powder has been slightly decreasing since a magnetic powder comprising iron oxide or iron oxide covered with cobalt has been developed.
This is because chromium dioxide contains Cr(IV) which is chemically unstable so that it is inevitable that the saturation magnetization of the chromium dioxide inherently deteriorates in the air with the passage of time.
However, the chromium dioxide powder has an excellent property such that when recorded information is written over, namely when the recorded information of the magnetic recording medium is erased for re-recording the information is satisfactorily erased. Since this good overwriting property of the chromium dioxide powder is suitable for a magnetic recording medium for use as the external memory of a computer, which should have good repeated recording properties, the chromium dioxide powder is being reevaluated.
Under these circumstances, some attempts have been made to suppress the deterioration of the saturation magnetization with the passage of time due to chemical instability of chromium dioxide while maintaining the inherent advantages of chromium dioxide.
The first attempt is to convert the surface layer of the chromium dioxide powder particle from dioxide to hydroxide of Cr(III) which is more stable than chromium dioxide and comprises treating the chromium dioxide powder with a reducing agent, such as sodium sulfite (cf. Italian Pat. No. 27333A/79 and U.S. Pat. Nos. 3,512,930 and 3,529,930).
The second attempt is to remove reactive materials such as adsorbed water present on the surface of the chromium dioxide powder particles and comprises heating the chromium dioxide powder at a temperature of 330.degree. to 370.degree. C. in a nitrogen atmosphere (cf. G. Bassile, Material Review Bulletin, 17, (1982) 1197).
However, the above attempts cannot prevent deterioration of the saturation magnetization of the chromium dioxide powder with the passage of time, and a saturation magnetic flux density of a magnetic recording medium comprising such modified or thermally treated chromium dioxide powder decreases by 10 to 30% from the original value after the recording medium is kept standing in the air at 60.degree. C. for 4 weeks.
This may be partly because the particle size of the chromium dioxide powder is made smaller to satisfy the requirements for high density recording and in turn its specific surface area increases so that reactivity of the particle surfaces increases and thereby the saturation magnetization more easily deteriorates than with the conventional chromium dioxide powder having a larger particle size, and partly because mutual reactivity between a binder and the chromium dioxide powder is increased by functional groups of an additive which is added to increase dispersibility of the chromium dioxide powder in the binder.
According to the study of the present inventors to find a good technique to prevent the deterioration of the saturation magnetization of the chromium dioxide powder with the passage of time, it was concluded that, by the above first attempt, although CrOOH is formed on the surface of the chromium dioxide powder particle, it may deteriorate the chemical stability of the chromium dioxide powder. Namely, according to the first attempt, even when CrOOH is formed on the surface of the chromium dioxide powder particle, it easily attracts water because of its hydrophilicity. This results in insufficient stability of the chromium dioxide powder.
By the above second attempt, the chromium dioxide powder is heated at a temperature of from 330.degree. to 370.degree. C. in an atmosphere of inert gas, such as nitrogen, to remove crystallization water of chromium dioxide or adsorbed water on the particle surface in order to increase the stability of chromium dioxide. However, the result is not satisfactory. This is because not only CrOOH but also chromium dioxide has strong bonding force with a reactive material such as water.